The technical field of this invention is low resolution Raman spectroscopy for invivo analysis of a sample, for example, analysis of blood vessels for atherosclerotic plaques using low resolution Raman spectroscopy.
It is well known that deposits of plaque on cardiovascular tissues, such as on the interior walls of arteries, can severely restrict or completely block the flow of blood. Plaques typically exists in two forms, namely, as calcified plaques or as fibrous plaques. Calcified plaques are more rigid and more difficult to remove than fibrous plaques. Previous studies suggest that plaque composition rather than the actual size or volume of the plaque determine acceleration of clinical symptoms (Loree et al. (1994) Artheroscler. Thromb. 14, 230-234). Thus, methods for detecting deposits of calcified plaque or fibrous plaque on blood vessels, and determining their composition, have substantial utility in the diagnosis and treatment of atherosclerosis and related cardiovascular ailments.
A variety of spectroscopic methods have been used to characterize arterial disease in situ (See e.g., Clarke et al. (1988) Lasers Surg. Med. 8: 45-59; Deckelbaum et al. (1995) Lasers Surg. Med. 16: 226-234; Yan et al. (1995) Lasers Surg. Med 16: 164-178; Manoharan et al. (1993) Artheroschlerosis 103: 181-193 and Baraga et al. (1992) Proc. Natl. Acad. Sci. 89: 3473-3477). By delivering excitation light and collecting emitted light through flexible optical fibers, fluorescence spectra from a coronary artery can be collected and used to differentiate normal tissue from abnormal tissue (Bartorelli et al. (1991) J. Am. Coll. Cardiol. 17:160B-168B and Richards-Kortrum et al. (1989) Am. Heart J. 118: 381-391). However, due to the limited difference in fluorescence spectra of chemical compounds, these spectra typically provide insufficient chemical composition information.
In contrast, Raman spectroscopic methods provide more detailed spectra capable of providing greater compositional information and an ability to differentiate normal from abnormal tissue. For example, Clarke et al. discuss using visible Raman spectroscopy to analyze the surface of diseased and healthy tissue sites on post-mortem specimens of calcified aortic valves and coronary artery segments (see Clarke et al. (1988) Lasers in Surgery and Medicine, 8, 45-49).
In Raman Spectroscopy of Atherosclerotic Plaque: Implications for Laser Angioplasty," Radiology, 177, 262 (1990), Redd et al. also disclose using visible Raman spectroscopy to analyze human cadaveric aorta, percutaneous peripheral atherectomy, and surgical endarterectomy samples and conclude that Raman spectroscopy allows fatty plaque to be distinguished from a normal artery.
Recently, Brennen et al. described using IR Raman spectroscopy to analyze the chemical composition of human coronary artery from homogenized coronary artery samples (Brennen et al. (1997) Applied Spec. 52; 201-20).
However, prior approaches to the use of Raman spectroscopy have been largely, if not entirely, limited to the characterization ex-vivo of specimens removed from the subject by excision or extraction. The limitation of Raman spectroscopy to post-surgical (or post-mortem) analysis was due to the large optical systems needed to obtain a high resolution spectrum.
Another drawback to IR Raman spectroscopy has been its expense of operation. A significant component of that expense is the laser system required to produce quality, high-resolution spectra. Even using a laser diode as the scattering source, the laser remains one of the major expenses in developing cost-effective Raman systems.
Thus, there exists a need for a low cost, simple Raman spectroscopic system for in-vivo analysis of a sample. Moreover, there exists a specific need for systems for analyzing the chemical components of atherosclerotic plaques in-vivo.